The present invention relates to novel adenosine receptor ligands of formula 
wherein
R1 is hydrogen, halogen or lower alkoxy; and
R2 is hydrogen; xe2x80x94C(O)-lower alkyl or xe2x80x94C(O)-phenyl, wherein the phenyl ring is unsubstituted or substituted by one or two substituents selected from the group consisting of halogen; lower alkyl; lower alkoxy or trifluoromethyl; or is xe2x80x94C(O)-furanyl or xe2x80x94C(O)-thiophenyl, wherein the rings are unsubstituted or substituted by halogen.
These compounds have useful pharmacological activities.
Adenosine modulates a wide range of physiological functions by interacting with specific cell surface receptors. The potential of adenosine receptors as drug targets was first identified in 1982. Adenosine is both structurally and metabolically related to the bioactive nucleotides adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and cyclic adenosine monophosphate (cAMP); to the biochemical methylating agent S-adenosyl-L-methione (SAM); and structurally to the coenzymes NAD, FAD and coenzym A; and to RNA. Together adenosine and these related compounds are important in the regulation of many aspects of cellular metabolism and in the modulation of different central nervous system activities.
The receptors for adenosine have been classified as A1, A2A, A2B and A3 receptors, which belong to the family of G protein-coupled receptors. Activation of adenosine receptors by adenosine initiates signal transduction mechanism. These mechanisms are dependent on the receptor associated G protein. Each of the adenosine receptor subtypes has been classically characterized by the adenylate cyclase effector system, which utilizes cAMP as a second messenger. The A1 and A3 receptors, coupled with Gi proteins, inhibit adenylate cyclase, leading to a decrease in cellular cAMP levels, while A2A and A2B receptors couple to Gs proteins, and activate adenylate cyclase, leading to an increase in cellular cAMP levels. It is known that the A1 receptor system includes the activation of phospholipase C and modulation of both potassium and calcium ion channels. The A3 receptor subtype, in addition to its association with adenylate cyclase, also stimulates phospholipase C and therefore activates calcium ion channels.
The A1 receptor (326-328 amino acids) was cloned from various species (canine, human, rat, dog, chick, bovine, guinea-pig) with 90-95% sequence identify among the mammalian species. The A2A receptor (409-412 amino acids) was cloned from canine, rat, human, guinea pig and mouse. The A2B receptor (332 amino acids) was cloned from human and mouse with 45% homology of human A2B with human A1 and A2A receptors. The A3 receptor (317-320 amino acids) was cloned from human, rat, dog, rabbit and sheep.
The A1 and A2A receptor subtypes play complementary roles in adenosine""s regulation of the energy supply. Adenosine, which is a metabolic product of ATP, diffuses from the cell and acts locally to activate adenosine receptors to decrease the oxygen demand by the A1 receptor or increase the oxygen supply by the A2A receptor so as to reinstate the balance of energy supply versus demand within the tissue. The actions of both subtypes are to increase the amount of available oxygen to tissue and to protect cells against damage caused by a short term imbalance of oxygen. One of the important functions of endogenous adenosine is to prevent damage during traumas such as hypoxia, ischaemia, hypotension and seizure activity.
Furthermore, it is known that the binding of the adenosine receptor agonist to mast cells expressing the rat A3 receptor resulted in increased inositol triphosphate and intracellular calcium concentrations, which activated antigen induced secretion of inflammatory mediators. Therefore, the A3 receptor plays a role in mediating asthmatic attacks and other allergic responses.
Adenosine is also a neuromodulator, possessing global importance in the modulation of molecular mechanisms underlying many aspects of physiological brain function by mediating central inhibitory effects. An increase in neurotransmitter release follows traumas such as hypoxia, ischaemia and seizures. These neurotransmitters are ultimately responsible for neural degeneration and neural death, which causes brain damage or death of the individual. The adenosine A1 agonists that mimic the central inhibitory effects of adenosine therefore are useful as neuroprotective agents. Adenosine has been proposed as an endogenous anticonvulsant agent, inhibiting glutamate release from excitory neurons and inhibiting neuronal firing. Adenosine agonists therefore are used as antiepileptic agents.
Adenosine antagonists stimulate the activity of the CNS and have proven to be effective as cognition enhancers. Selective A2a-antagonists have therapeutic potential in the treatment of various forms of dementia, for example in Alzheimer""s disease and are useful as neuroprotective agents. Adenosine A2-receptor antagonists inhibit the release of dopamine from central synaptic terminals and reduce locomotor activity and consequently improve Parkinsonian symptoms. The central activities of adenosine are also implicated in the molecular mechanism underlying sedation, hypnosis, schizophrenia, anxiety, pain, respiration, depression and substance abuse. Drugs that modulate the adenosine receptors therefore also have therapeutic utility such as sedatives, muscle relaxants, antipsychotics, anxiolytics, analgesics, respiratory stimulants and antidepressants.
An important role for adenosine in the cardiovascular system is as a cardioprotective agent. Levels of endogenous adenosine increase in response to ischaemia and hypoxia, and protect cardiac tissue during and after trauma (preconditioning). Adenosine agonists thus have potential as cardioprotective agents.
Adenosine modulates many aspects of renal function, including renin release, glomerular filtration rate and renal blood flow. Compounds, which antagonize the renal affects of adenosine, are useful as renal protective agents. Furthermore, adenosine A3 and/or A2B antagonists are useful in the treatment of asthma and other allergic responses.
Numerous documents describe the current knowledge on adenosine receptors. These include Bioorganic and Medicinal Chemistry, 6, (1998), 619-641; Bioorganic and Medicinal Chemistry, 6, (1998), 707-719; J. Med. Chem., (1998), 41, 2835-2845, J. Med. Chem., (1998), 41, 3186-3201; J. Med. Chem., (1998), 41, 2126-2133; J. Med. Chem., (1999), 42, 706-721; J. Med. Chem., (1996), 39, 1164-1171; and Arch. Pharm. Med. Chem., (1999), 332, 39-41. The first two of these references disclose the agonist and antagonist functions of each of the receptor subtypes (A1, A2A, A2B, and A3) and their physiological effects. For example, adenosine receptor (AR) antagonists with selectivity for the A1-AR is useful for the treatment of senile dementia such as Alzheimer""s disease and for the prevention of acute renal failure. A2A-selective antagonists are useful for the treatment of Parkinson""s disease, hypotension and ischemias. A2B- and A3-selective antagonists are useful for the treatment of asthma and other allergic responses.
An aspect of the present invention is directed to the compounds of formula 
wherein
R1 is hydrogen, halogen or lower alkoxy; and R2 is hydrogen; xe2x80x94C(O)-lower alkyl or xe2x80x94C(O)-phenyl, wherein the phenyl ring is unsubstituted or substituted by one or two substituents selected from the group consisting of halogen; lower alkyl; lower alkoxy or trifluoromethyl;
or is xe2x80x94C(O)-furanyl or xe2x80x94C(O)-thiophenyl, wherein the rings are unsubstituted or substituted by halogen;
and their pharmaceutically acceptable salts. Other embodiments of the invention are directed to methods of manufacture of compounds of formula I, pharmaceutical compositions containing a compound of formula I or a pharmaceutically acceptable salt thereof, as well as a method of controlling or preventing of illnesses based on the modulation of the adenosine system, such as Alzheimer""s disease, Parkinson""s disease, neuroprotection, schizophrenia, anxiety, pain, respiration deficits, depression, asthma, allergic responses, hypoxia, ischaemia, seizure and substance abuse comprising administering to a patient a compound of formula I.
Furthermore, the compounds of the present invention are useful as sedatives, muscle relaxants, antipsychotics, antiepileptics, anticonvulsants and cardiaprotective agents. Preferred indications in accordance with the present invention are those that depend on A2A receptor antagonistic activity and include disorders of the central nervous system, for example, the treatment or prevention of certain depressive disorders, neuroprotection and Parkinson""s disease.
As used herein, the term xe2x80x9clower alkylxe2x80x9d refers to a saturated straight- or branched-chain alkyl group containing from 1 to 6 carbon atoms, for example, methyl, ethyl, propyl, isopropyl, n-butyl, i-butyl, 2-butyl, t-butyl and the like. Preferred lower alkyl groups are groups with 1 to 4 carbon atoms.
The term xe2x80x9chalogenxe2x80x9d refers to chlorine, iodine, fluorine and bromine.
The term xe2x80x9clower alkoxyxe2x80x9d refers to a group wherein the alkyl residues is as defined above, and which is attached via an oxygen atom.
The term xe2x80x9cpharmaceutically acceptable acid addition saltsxe2x80x9d refers to salts with inorganic and organic acids, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, citric acid, formic acid, fumaric acid, maleic acid, acetic acid, succinic acid, tartaric acid, methane-sulfonic acid, p-toluenesulfonic acid and the like.
Compounds of formula I of the present invention, wherein R2 is xe2x80x94C(O)-phenyl, substituted by halogen, are preferred. For example, these compounds include:
4-Fluoro-N-(5-methoxy-8-phenyl-[1,2,4]triazolo[1,5-a]pyridin-2-yl)-benzamide;
4-bromo-N-(5-methoxy-8-phenyl-[1,2,4]triazolo[1,5-a]pyridin-2-yl)-benzamide;
4-bromo-N-[5-methoxy-8-(3-methoxy-phenyl)-[1,2,4]triazolo[1,5-a]pyridin-2-yl]-benzamide;
4-fluoro-N-[8-(4-fluoro-phenyl)-5-methoxy-[1,2,4]triazolo[1,5-a]pyridin-2yl]-benzamide; and
4-fluoro-N-[5-methoxy-8-(3-methoxy-phenyl)-[1,2,4]triazolo[1,5-a]pyridin-2-yl]-benzamide.
Another preferred set of compounds of formula I of the present invention include those where R2 is xe2x80x94C(O)-furanyl, substituted by halogen. Examples of this group include:
5-Bromo-furan-2-carboxylic acid [8-(3-fluoro-phenyl)-5-methoxy-[1,2,4]triazolo[1,5-a]pyridin-2-yl]-amide; and
5-bromo-furan-2-carboxylic acid [5-methoxy-8-(3-methoxy-phenyl)-[1,2,4]triazolo[1,5-a]pyridin-2-yl]-amide.
Yet another preferred set of compounds of formula I of the present invention include that where R2 is xe2x80x94C(O)-thiophenyl. An example of these compounds includes Thiophene-2-carboxylic acid [5-methoxy-8-(3-methoxy-phenyl)-[1,2,4]triazolo[1,5-a]pyridin-2-yl]-amide.
The present compounds of formula I and their pharmaceutically acceptable salts can be prepared by methods known in the art, for example:
a) reacting a compound of formula 
with ethoxycarbonyl isothiocyanate to form a compound of formula 
and cyclizing the compound of formula III in the presence of hydroxylamine to form a compound of formula 
wherein R1 has the significance given above, or
reacting a compound of formula 
with a compound of formula
R2Cl
to form a compound of formula 
wherein
R1 is hydrogen, halogen or lower alkoxy; and
R2 is hydrogen; xe2x80x94C(O)-lower alkyl or xe2x80x94C(O)-phenyl, wherein the phenyl ring is unsubstituted or substituted by one or two substituents selected from the group consisting of halogen; lower alkyl; lower alkoxy or trifluoromethyl; or is xe2x80x94C(O)-furanyl or xe2x80x94C(O)-thiophenyl, wherein the rings are unsubstituted or substituted by halogen
and
if desired, converting the compounds obtained into pharmaceutically acceptable salts.
The preparation of compounds of formula I is described in more detail based on Scheme 1 and Examples 1-42 below:
As used herein, DIPEA in scheme 1 refers to N-ethyldiisopropyl-amine. 
In accordance with scheme 1, the compound of formula V (6-amino-5-bromo-pyridin-2-ol) may be prepared as described in Kelly, T. R.; Jagoe, C. T.; Gu, Z. Tetrahedron Letters 1991, 32, 4263-4266) as follows: To a solution of 6-amino-pyridin-2-ol in acetic acid at room temperature is added bromine and stirred for 15 min. The mixture is diluted with water and the precipitate is filtered off. The filtrate is extracted and the combined organic layers are dried and evaporated to dryness. Then a suspension of 6-amino-5-bromo-pyridin-2-ol is treated with KOH pellets and dimethylsulfate. The mixture is stirred for 4 h at room temperature and evaporated to dryness. The residue is purified and 3-bromo-6-methoxy-pyridin-2-yl-amine (IV) is obtained. Furthermore, a mixture of 3-bromo-6-methoxy-pyridin-2-yl-amine, phenylboronic acid (wherein the phenyl ring may be substituted by R1), Na2CO3 and dichloro[1,1xe2x80x2-bis(diphenylphosphino)-ferrocene]palladium II) dichloromethane adduct in dioxane is heated to 110xc2x0 C. for 2 h. The mixture is concentrated, diluted Na2CO3 aq. is added and extracted. The combined organic phases are dried and evaporated. The residue is purified to yield the corresponding compound of formula II, for example 6-methoxy-3-phenyl-pyridin-2-yl-amine. A mixture 6-methoxy-3-phenyl-pyridin-2-yl-amine (II) and ethoxycarbonyl isothiocyanate is stirred at room temperature for 2 h and afterwards evaporated to dryness. The obtained compound of formula III is then treated with a mixture of hydroxylamine hydrochloride and N-ethyldiisopropylamine (DIPEA). The mixture is heated to 80xc2x0 C. for 16 h, evaporate to dryness, taken up in water and extracted with diethyl ether. The combined organic phases are dried and evaporated to yield, for example, 5-methoxy-8-phenyl-[1,2,4]triazolo[1,5-a]pyridin-2-yl-amine (Ia). A mixture of 5-methoxy-8-phenyl-[1,2,4]triazolo[1,5-a]pyridin-2-yl-amine and a compound of formula R2Cl, for example 3-fluorophenyl carboxylic acid chloride, and NEt3 in dioxane is heated to 90xc2x0 C. for 16 h. The mixture is purified to give a compound of formula I, for example 3-fluoro-N-(5-methoxy-8-phenyl-[1,2,4]triazolo[1,5-a]pyridin-2-yl)-benzamide.
The salt formation is effected at room temperatures in accordance with methods which are known and familiar to any person skilled in the art. The salts contain inorganic acids as well as organic acids. Hydrochlorides; hydrobromides; sulphates; nitrates; citrate; acetates; maleates; succinates; methan-sulphonates; p-toluenesulphonates and the like are examples of such salts.
The compounds of formula I and their pharmaceutically usable addition salts possess valuable pharmacological properties. Specifically, it has been found that the compounds of the present invention are adenosine receptor ligands that modulate receptor function, particularly noting that each of their subtypes (A1, A2A, AZB and A3) have defined agonist and antagonist effects.
The compounds were investigated in accordance with the following standard assays.
The human adenosine A2A receptor was recombinantly expressed in Chinese hamster ovary (CHO) cells using the semliki forest virus expression system. Cells were harvested, washed twice by centrifugation, homogenized and again washed by centrifugation. The final washed membrane pellet was suspended in a Tris (50 mM) buffer containing 120 mM NaCl, 5 mM KCl, 2 mM CaCl2 and 10 mM MgCl2 (pH 7.4) (buffer A). The [3H]-SCH-58261 (Dionisotti et al., 1997, Br. J. Pharmacol. 121, 353) binding assay was carried out in 96-well plates in the presence of 2.5 xcexcg of membrane protein, 0.5 mg of Ysi-poly-l-lysine SPA beads and 0.1 U adenosine deaminase in a final volume of 200 xcexcl of buffer A. Non-specific binding was defined using xanthine amine congener (XAC; 2 xcexcM). Compounds were tested at 10 concentrations from 10 xcexcM to 0.3 nM. All assays were conducted in duplicate and repeated at least twice. Assay plates were incubated for 1 hour at room temperature before centrifugation and then bound ligand was determined using a Packard Topcount scintillation counter. IC50 values were calculated using a non-linear curve fitting program and Ki values calculated using the Cheng-Prussoff equation.
In accordance with the invention, the compounds of formula I have a high affinity toward the A2A receptor. The table below (after Example 10) describes specific values of prepared compounds.
The compounds of formula I and the pharmaceutically acceptable salts of the compounds of formula I can be used as medicaments, e.g., in the form of pharmaceutical preparations. The pharmaceutical preparations can be administered orally, e.g., in the form of tablets, coated tablets, dragees, hard and soft gelatine capsules, solutions, emulsions or suspensions. The administration can, however, also be effected rectally, e.g., in the form of suppositories, or parenterally, e.g., in the form of injection solutions.
The compounds of formula I can be processed with pharmaceutically inert, inorganic or organic carriers for the production of pharmaceutical preparations. Lactose, corn starch or derivatives thereof, talc, stearic acids or its salts and the like can be used, for example, as such carriers for tablets, coated tablets, dragees and hard gelatine capsules. Suitable carriers for soft gelatine capsules are, for example, vegetable oils, waxes, fats, semi-solid and liquid polyols and the like. Depending on the nature of the active substance, no carriers are usually required in the case of soft gelatine capsules. Suitable carriers for the production of solutions and syrups are, for example, water, polyols, glycerol, vegetable oil and the like. Suitable carriers for suppositories are, for example, natural or hardened oils, waxes, fats, semi-liquid or liquid polyols and the like.
The pharmaceutical preparations can, moreover, contain preservatives, solubilizers, stabilizers, wetting agents, emulsifiers, sweeteners, colorants, flavorants, salts for varying the osmotic pressure, buffers, masking agents or antioxidants. They can also contain still other therapeutically valuable substances.
Medicaments containing a compound of formula I or a pharmaceutically acceptable salt thereof and a therapeutically inert carrier are also a part of the present invention, as is a process for their production, which comprises bringing one or more compounds of formula I and/or pharmaceutically acceptable acid addition salts and, if desired, one or more other therapeutically valuable substances into a galenical administration form together with one or more therapeutically inert carriers.
In accordance with the invention compounds of formula I as well as their pharmaceutically acceptable salts are useful in the control or prevention of illnesses based on the adenosine receptor antagonistic activity, such as Alzheimer""s disease, Parkinson""s disease, neuroprotection, schizophrenia, anxiety, pain, respiration deficits, depression, asthma, allergic responses, hypoxia, ischaemia, seizure and substance abuse. Furthermore, compounds of the present invention may be useful as sedatives, muscle relaxants, antipsychotics, antiepileptics, anticonvulsants and cardiaprotective agents and for the production of corresponding medicaments.
The most preferred indications in accordance with the present invention are those, which include disorders of the central nervous system, for example, the treatment or prevention of certain depressive disorders, neuroprotection and Parkinson""s disease.
The dosage can vary within wide limits and will, of course, have to be adjusted to the individual requirements in each particular case. In the case of oral administration, the dosage for adults can vary from about 0.01 mg to about 1000 mg per day of a compound of formula I or of the corresponding amount of a pharmaceutically acceptable salt thereof. The daily dosage may be administered as single dose or in divided doses and, in addition, the upper limit can also be exceeded when this is found to be indicated.